Technical Field
The present disclosure relates generally to switching regulators, and more specifically, to methods, circuits and systems for maximum current threshold control of a switching regulator that maintains stability and reduces the occurrences of interruptions of in the normal switching operation of the switching regulator.
Description of the Related Art
A voltage regulator provides a regulated output voltage to a load from a voltage source that may be poorly regulated or fluctuates. A switching voltage regulator provides not continuous current from the voltage source to a load but instead provides current pulses from the voltage source to the load. The voltage regulator includes a switching circuit, typically including at least one power transistor, coupled to the load and this switching circuit is controlled to alternately store electrical energy in and discharge electrical energy from an inductive element. This electrical energy stored in and discharged from the inductive element is utilized to generate the regulated output voltage that is supplied to the load.
The switching circuit has a switching cycle SC that includes a portion during which the switching circuit is turned ON and a portion during which the switching circuit is turned OFF. When the switching circuit is turned ON, energy from the voltage source is stored in the inductive element and when the switching circuit is turned OFF, energy is discharged from the inductive element. The duty cycle D of the voltage regulator is defined as the fraction of the switching cycle for which the switching circuit is turned ON, and is given by the time the switching circuit is turned ON divided by the period of the switching cycle. The switching voltage regulator controls the duty cycle D to thereby regulate the load or output voltage supplied to the load.
A switching voltage regulator typically includes two control loops for controlling the operation of the regulator. A voltage control loop generates a control voltage responsive to the value of the output voltage while an inner current control loop adjusts a peak current flowing through the inductive element based on the control voltage. The terms inductive element and inductor are used interchangeably in the present description to mean any suitable type of inductive circuit such as a single inductor, multiple inductors, a transformer, and so on. This current-mode control implemented by the current control loop typically detects a peak current through the inductor and turns OFF the switching circuit when the current reaches this peak current.
When peak current mode control is utilized in controlling the operation of a switching voltage regulator, an instability in the operation of the regulator inherently exists due to sub-harmonic oscillations when the duty cycle D of the regulator exceeds 50% (i.e., 0.5), as will be appreciated by those skilled in the art. Due to this inherent instability, when the duty cycle D exceeds 50% a current threshold for the peak current mode control has a value that is a function of the current through the inductor and a compensation signal. This compensation signal has rate of change or slope related to the rate of change of the current through the inductor each switching cycle and is accordingly referred to as slope compensation.
In addition to the just described peak current mode control, the current control loop of a switching voltage regulator also includes current limit control that controls switching if the current through the inductor exceeds a maximum current threshold. This functions to protect the inductor and regulator from damage that could result from allowing the current through the inductor to exceed this maximum current threshold. Large inductor currents could result, for example, where an overload condition such as a short circuit occurs across the load being driven by the switching voltage regulator. Conventional approaches to this maximum current threshold detect each switching cycle whether the inductor current exceeds the maximum current threshold for a number of switching cycles and, if so, then performing a restart of the voltage regulator.
The restart includes a period of time during which the switching operation of the voltage regulator is terminated followed by a “soft-start” of the voltage regulator. This soft-start is a mode of operation of the voltage regulator that controls currents flowing in the regulator during restart to prevent the relatively large currents that would otherwise flow during restart. For example, during restart of a switching voltage regulator a relatively large current could be demanded from the input voltage source of the regulator without this soft-start mode of operation. The soft-start mode prevents this from happening by gradually increasing the permissible current limit through the inductor over time during the soft-start mode.
While these conventional approaches utilizing restarts including the soft-start mode of operation do perform maximum current threshold control, there are undesirable consequences that result. One undesirable consequence resulting from a soft-start is that this soft-start mode of operation takes a relatively long time as the current allowed to flow through the inductor is gradually ramped up to its normal maximum permissible value. During most of this time the output voltage from the switching voltage regulator is not being regulated as desired and therefore can undesirably fluctuate, which could adversely affect the operation of electronic circuitry in the load being driven by the voltage regulator. Other undesirable consequences of such restarts are relatively high power dissipation in certain components of the regulator during an overload condition, necessitating certain components be oversized or the size of associated heat sinks increased accordingly. A technique known as foldback current limiting by which the maximum current limit threshold is reduced as the output voltage falls during an overload condition may also be utilized but this can result in unpredictable operation of the regulator, as will be understood by those skilled in the art. Accordingly, there is a need for improved techniques of performing maximum current threshold control in switching voltage regulators.